The present invention concerns a spring steel of medium strength having good corrosion-resistance. The steel of the invention is particularly suitable for material of automobile suspension system.
In order to meet the demand for light-weighting of automobiles, it is necessary to light-weight springs of suspension systems of the automobiles, and therefore, there is demand for a spring steel having high resistance to permanent set in fatigue. There has been proposed so-called "high-silicon spring steel" prepared by adding to a spring steel which contains as main alloying elements, C: 0.35-0.45%, Si: 1.50-2.50% and Mn: 0.50-1.50% with the balance of Fe, at least one of V, Nb and Mo in a suitable amount or amounts to form a carbide or carbides (Japanese Patent Disclosure No. 58-67847). This steel may further contain one or both of the elements of two groups: one or more of Ti, Al and Zn in a suitable amount or amounts; and one or more of B, Cr, Ni and REM in a suitable amount or amounts.
The applicant has developed and proposed high strength spring steels (Japanese Patent Disclosures Nos. 63-109144 and 63-216951). These steel are also of high-Si content (1.0-4.0%) and contains Cr: 0.1-2.0% and Ni: up to 2.0% in addition to C: 0.3-0.75% and Si:1.0-4.0%, and characterized in that occurence of retained austenite after quenching is less than 10%. In order to keep the retained austenite occurence less than 10%, contents of C, Si and Ni are chosen to such amounts that satisfy the inequality: 35.multidot.C %+2.multidot.Si % +Ni %&lt;23%. This steel may further contain suitable amounts of V and/or Mo.
Separate to these steels, the applicant also developed a spring steel having high corrosion-resistance and corrosion-fatigue strength, and disclosed it (Japanese Patent Disclosure No. 02-301541). The steel exhibits high corrosion-resistance by forming direct oxide layers of thickness of 20 micrometers or thicker on the surface of the spring products. Due to the alloy composition of this steel similar to those of stainless steels, i.e., contents of Cr: 3-5% and Ni: 1-2%, costs of the steel products are somewhat high. Further, processability in the secondary processing of this steel is not so good.
A spring steel of such a high tensile strength as 200 kgf/mm.sup.2 was proposed (Japanese Patent Disclosure No. 05-320826). This high tensile strength is achieved by adjusting hardness after quenching-tempering to HRC 53 or higher.
The high strength spring steel first mentioned in this description of the invention which was developed by the applicant is designed to have such a relatively high stress such as 130 kgf/mm.sup.2. To produce wire rods for springs from this steel, it is necessary to go through the steps of: rolling--spheroidizing annealing--wire drawing--grinder abrasion. Because of relatively high alloying composition and necessity of heat treatment, costs for producing wire rods from this spring steel are considerably high in comparison with those for producing the conventional spring steel rods. Thus, there has been demand for a spring steel designed to have a strength level of 120 kgf/mm.sup.2 with a lower alloying composition and simplified process for producing wire materials, and consequently, of lowered costs. This spring steel, which is used mainly for automobile suspension systems, should have, in addition to high resistance to permanent set in fatigue, excellent fatigue properties under corrosive environments. It is preferable that the steel can easily be processed in secondary processing steps, more specifically, that hardness as rolled is low.